Sorption processes to remove contaminants from water are operationally simple, require virtually no start-up time, and are forgiving toward fluctuations in feed compositions. A viable and economically competitive sorbent should exhibit high selectivity toward the target contaminant, should be durable, and should be amenable to efficient regeneration and reuse. Removing the target contaminant should not cause major changes in pH or in the composition of the influent water.
Sorbents that contain at least one oxygen-containing compound of a metal, such as amorphous and crystalline hydrated iron (Fe) oxide compounds (HFO), may have these qualities. Such sorbents show strong sorption affinity toward both arsenic (III) and arsenic (V) species in solution. HFO particles also show strong sorption affinity towards phosphate, natural organic matter (NOM), selenite, molybdate, vanadate, arsenite, arsenate, phosphate, and other ligands. Other competing ions, such as chloride or sulfate, exhibit poor sorption affinity toward HFO particles.
Traditional synthesis processes of HFO produce only very fine (e.g., micron-sized) HFO particles. Such fine HFO particles are unusable in fixed beds, permeable reactive barriers, or any flow through systems because of excessive pressure drops, poor mechanical strength, and unacceptable durability. To overcome the problem of very fine HFO particles, strong-acid cation exchangers have been modified to contain HFO particles. These supported HFO particles are useful for the removal of arsenic and other contaminants.
Iron loaded cation exchange resins, complexing resins, and alginates have also been tried to remove selenium and arsenic oxyanions. Although cation exchanger loaded HFO particles are capable of removing arsenates or phosphates, their removal capacities are reduced because the cation exchange material is negatively charged due to sulfonic acid or other negatively charged functional groups. The HFO particles dispersed in the cation exchange material are not accessible to dissolved anionic ligands for selective sorption. Consequently, arsenates, phosphates and other oxyanions are rejected due to the Donnan co-ion exclusion effect.
Macroporous cation exchange sorbents with dispersed HFO particles provided arsenic sorption capacity of about 750 μg/g sorbent. Gel-type cation exchange sorbents with dispersed HFO particles provided minimal arsenic sorption capacity; a gel-type cation exchange sorbent loaded with eight percent iron resulted in almost immediate arsenic breakthrough. HFO particles encapsulated with cation exchange sites were not accessible to arsenates or other anionic ligands for selective sorption.
Accordingly, there is a need for a more effective medium and method for selective removal of contaminants from fluid streams, and a method for effectively dispersing HFO particles throughout anion exchange materials.